1. Field of the Invention
The present invention relates to a magnetic detector, a thin-film magnetic head, a head assembly and a magnetic disk drive each of which includes a magnetoresistive element and an impact sensor.
2. Description of the Related Art
A magnetic disk drive includes a recording medium to be driven to rotate, and a thin-film magnetic head for writing data on and reading data from the recording medium. The thin-film magnetic head for use in the magnetic disk drive is typically in the form of a slider with a thin-film magnetic head element (hereinafter, also simply referred to as a head element) provided at a rear end thereof. In the magnetic disk drive the slider is flexibly supported by a suspension so as to face toward the recording medium. In the magnetic disk drive, when the recording medium rotates, an airflow passes between the recording medium and the slider and causes a lift to be exerted on the slider. This lift causes the slider to slightly fly over the surface of the recording medium.
Typically, a head element includes a magnetoresistive element (hereinafter, also referred to as an MR element) for reading and an induction-type electromagnetic transducer for writing. Examples of the MR element include a giant magnetoresistive (GMR) element utilizing a giant magnetoresistive effect, and a tunneling magnetoresistive (TMR) element utilizing a tunneling magnetoresistive effect.
A spin-valve GMR element and a TMR element each typically include a free layer, a pinned layer, a spacer layer disposed between the free layer and the pinned layer, and an antiferromagnetic layer disposed on a side of the pinned layer farther from the spacer layer. The free layer is a ferromagnetic layer whose magnetization direction changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer whose magnetization direction is fixed. The antiferromagnetic layer is a layer that fixes the magnetization direction of the pinned layer by means of exchange coupling with the pinned layer. The spacer layer is a nonmagnetic conductive layer in a spin-valve GMR element, and is a tunnel barrier layer in a TMR element.
In a magnetic disk drive, if there are protrusions on the surface of the recording medium caused by, for example, adhesion of foreign substances or defects of the recording medium, the magnetic head may collide with the protrusions. It is difficult to perform proper reading and writing in portions of the recording medium with which the magnetic head collides. To cope with this, such measures are taken that portions of the recording medium with which the magnetic head collides are detected in advance, and the operation of the magnetic disk drive is controlled so as to prevent the magnetic head from passing over those portions and/or a pass or fail of the recording medium is determined based on the number of protrusions with which the magnetic head collides.
As disclosed in U.S. Pat. No. 5,793,207, it is known that a collision of a magnetic head with a recording medium causes a sudden increase in temperature of the MR element. Accordingly, in a case of a magnetic head using an MR element having a high resistance-temperature coefficient, that is, a high ratio of change in resistance with respect to a change in temperature, it is possible to detect a collision of the magnetic head with the recording medium by detecting a sudden change in resistance of the MR element associated with a sudden increase in temperature of the MR element. However, for a magnetic head using an MR element having a low resistance-temperature coefficient such as a TMR element, it is difficult to detect a collision of the magnetic head with the recording medium through the above-described method.
The free layer of an MR element typically has a non-zero magnetostriction constant. Consequently, when the magnetic head suffers an impact due to a collision with the recording medium and distortion thereby occurs in the free layer, the magnetization direction of the free layer changes by an inverse magnetostrictive effect. Hence it is possible to detect a collision of the magnetic head with the recording medium by detecting a change in resistance of the MR element occurring upon a change in magnetization direction of the free layer. However, this method has the following problem.
In an MR element, typically, the magnetization direction of the free layer is in a specific direction such as the track width direction when the free layer is in a state in which a bias magnetic field is applied but no signal magnetic field is applied thereto. Accordingly, in order for the free layer to resist the bias magnetic field and thereby exhibit a great change in magnetization direction by the inverse magnetostrictive effect when the magnetic head suffers an impact, it is necessary that the free layer have a magnetostriction constant having a large absolute value. However, if the free layer has a magnetostriction constant having a large absolute value, the magnetic anisotropy of the free layer greatly varies depending on the magnitude of stress that the layers constituting the MR element or the layers neighboring the MR element inherently have, and/or the magnitude of stress generated in the layers constituting the MR element or the layers neighboring the MR element due to a change in temperature of the magnetic head. Therefore, the free layer having a magnetostriction constant having a large absolute value is undesirable for the characteristics of the MR element although it is desirable in detecting an impact on the magnetic head.
Another possible approach is to provide a sensor in the magnetic head separate from the MR element, for detecting a collision of the magnetic head with the recording medium. However, this requires a process dedicated to forming the sensor in the manufacture of the magnetic head and thereby makes the manufacturing process for the magnetic head more complicated.
U.S. Patent Application Publication No. 2008/0211490 A1 discloses a magnetic detector including an MR element and a fixed resistor, the fixed resistor having a resistance that does not change in response to an external magnetic field. The MR element and the fixed resistor are formed of the same laminates on one substrate and are connected to each other in series. In this magnetic detector, hard bias layers are provided with spacing on both sides of the fixed resistor, and the magnetization direction of a magnetic layer of the fixed resistor corresponding to the free layer of the MR element is fixed by a bias magnetic field sent from the hard bias layers. As a result, even if the magnetic head suffers an impact, the magnetization direction of the magnetic layer of the fixed resistor corresponding to the free layer will not change easily. It is therefore difficult with the fixed resistor of this magnetic detector to detect an impact on the magnetic head.